1. Field of the Invention
This invention relates to modem systems, and more particularly to a modem system in which a PSK modulation is used for transmission of digital data and, on the signal receiving side, delay demodulation is used to obtain the original digital data.
2. Brief Discussion of the Art
In a conventional PSK digital signal transmission system, a signal to be transmitted is PSK-modulated on the signal transmitting side and, on the signal receiving side, the signal is demodulated to obtain the original digital data.
In PSK modulation systems, the digital signals "0" and "1" are transmitted in correspondence to the phase of a carrier, and the C/N ratio deterioration is minimized. Because of its excellent signal characteristics, the PSK modulation system is extensively employed for transmission of digital signals.
The PSK modulation system will be described in more detail. Usually one of a so-called MSK system, as shown in FIG. 7, or a so-called DSK system, as shown in FIG. 8, are employed for PSK modulation.
In the MSK system, when a signal is in "mark" state, the phase is linearly increased by 180.degree. for one time slot period of the signal; and when the signal is in "space" state, the phase is linearly decreased by 180.degree. for one time slot period.
In the DSK system, when a signal is in "mark" state, the phase is increased in two steps, i.e., one time slot period of the signal is divided into two parts and, in each part, the phase is increased by 90.degree.; and when the signal is in "space" state, the phase is decreased in two steps, i.e., in each part, the phase is decreased by 90.degree..
The MSK system is advantageous in that the occupied bandwidth is small because the phase changes continuously. The DSK system is suitable for wideband data transmission, because it is substantially free from multi-path fading.
In general, a delay detection system or a synchronous detection system are employed for demodulation of the signals which are PSK-modulated as described above.
In the delay detection system, a signal received is divided into two parts. One part is applied directly to a phase comparator, while the other is supplied through a delay circuit to the phase comparator. In the delay circuit, the signal is delayed by one (1) or half (1/2) signal period of the modulation. Thus, by comparing the relatively delayed signals the PSK-modulated signal is demodulated into the original digital signal. This will be described in more detail with reference to FIGS. 5A, 5B and 5C.
It is assumed that, in a delay detection device shown in FIG. 5A, its input voltage is represented by the following equation: EQU V.sub.in =cos (.OMEGA.t+.theta.(t))
where .theta. is the angular frequency of the carrier, t is the time, and .theta.(t) is the phase modulation function. The input voltage V.sub.in is divided into two parts. One of the two parts is supplied to one input terminal of a phase comparator 22, and the other, after being delayed by a predetermined period of time TR by a delay circuit 21, is applied to the other input terminal of the phase comparator 22. Therefore, the signal V.sub.c applied to the one input terminal of the phase comparator is: EQU V.sub.c =V.sub.in =cos (.OMEGA..sub.t +.theta.(t),
while the signal V.sub.d supplied to the other input terminal of the phase comparator is: EQU V.sub.d =cos (.OMEGA.(t-TR)+.theta.(t-TR))
In the case where the phase comparator 22 is as shown in FIG. 5B; that is, the phase comparator 22 is such that the output is proportional to the phase difference of the input signals, then the phase difference is: EQU .DELTA..theta.=.OMEGA.TR+.theta.(t)-.theta.(t-TR)
In the MSK system or in the DSK system, the delay time TR should be equal to T/2 (where T is one time slot of the signal).
With EQU .OMEGA.TR=(2n-1).pi.
or EQU .OMEGA.=.pi./TR 32 (2n-1)2.pi./T,
a reference point for phase comparison can be set at the center of the range of operation of the phase comparator.
By way of further example, a DSK modulation system will be described in more detail; however, the description is applicable to the case of the MSK system as well.
When EQU .theta.(t)-.theta.(t-TR)=0, EQU .DELTA..theta.=.OMEGA.TR=(2n-1).
Therefore, this point is the phase reference point in the case where there is no phase shift, and, the output provided corresponds to the point shifted by .theta.(t)-.theta.(t-TR) from that point.
In the case of a mark-space signal, the phase function .theta.(t) is as indicated in FIG. 6A, and the function .theta.(t-T/2) in FIG. 6B.
Accordingly, as shown in FIG. 6C, .theta.(t)-.theta.(t-T/2) is .pi./2 for a "mark" period and a -.pi./2 for a "space" period, and an output waveform as shown in FIG. 6E is obtained according to an output characteristic indicated in FIG. 6D. That is, the output is 3 V.sub.0 /4 for a "mark" period and V.sub.0 /4 for a "space" period.
Therefore, it can be determined that the signal is the "mark" when the output of the phase comparator 22 exceeds V.sub.0 /2, and it is the "space" when the output is V.sub.0 /2 or less.
In a synchronous detection circuit, an input signal is divided into two parts which are supplied to two phase comparators, respectively, and the output signal (having a frequency coincident with the carrier frequency of the input signal) of a voltage-controlled oscillator included in a phase synchronization loop is applied, as it is, to one of the phase comparators, while the output signal, after being shifted 90.degree. in phase, is supplied to the other phase comparator, so that the original digital signal is obtained by utilization of the output signals of the two phase comparators (cf. Trans. IECE Japan, Vol. 64-B, No. 10, 1981, GMSK Modulation System Transmission Characteristic by Kazuaki Murota and Kenkichi Hiraide).
As discussed above, when PSK-modulated signal is demodulated according to the above-described delay detection system, the signal received is divided into two parts, one of which is merely delayed. Therefore, the circuitry can be considerably simplified. However, in the case where the PSK modulation is applied to the transmission of a digital signal in a high frequency band, a difficulty is involved in that the demodulation reliability is rather low.
This will be described in more detail. In the delay detection system, the operating reference point is: EQU .DELTA..theta.=.OMEGA.T/2
Therefore, as the carrier angular frequency changes as much as .DELTA..OMEGA. by a temperature variation, the operating reference point is shifted as much as .DELTA..OMEGA.T/2. If this change is great, then it is impossible to determine the "mark" and "space" periods according to whether or not the output level of the phase comparator exceeds V.sub.0 /2. For instance when the carrier frequency is 2.5 GHz, and the temperature variation coefficient of the oscillator (which is a SAW oscillator in this case) is a .+-.3.times.10.sup.-4, then the frequency variation is .+-.750 KHz. If, in this case, the data transmission speed is set to 32 K bps, then T=1/32 msec, and .DELTA..OMEGA.T/2 =23.44 .sub..pi. ; that is, the change of the operating reference point is about 23.44 .sub..pi.. In practice, the operating reference point is further shifted, being affected not only by temperature variation, but also by noise and interference waves by multipath. Therefore, it is often difficult or impossible to determine the "mark" and "space" periods through comparison of the input level of the phase comparator with a predetermined reference level.
The synchronous detection system is based on the reproduction of the carrier frequency utilizing a COSTAS loop. The synchronous detection system, unlike the phase detection system, is free from the difficulties attributed to frequency variation, and can therefore demodulate the signal with high accuracy.
However, the synchronous detection system still suffers from the following problems: Provision of a signal having a frequency equal to the carrier frequency of a signal received requires a local oscillator, namely, a voltage-controlled oscillator, and a phase synchronization loop, which not only makes the circuitry intricate, but also increases the manufacturing cost. This difficulty is serious for a radio device installed on a vehicle, because emphasis is placed on miniaturization, simplification and cost reduction of the radio device.
In an effort to improve reliability of data detection using a delay detection device, applicants proposed in Japanese Application No. 165014/86, filed July 14, 1986 (corresponding to U.S. Ser. No. 072,162, entitled "PSK Modem System Having Improved Demodulation Reliability", filed July 10, 1987, the disclosure of which is incorporated herein by reference in its entirety), a modem system in which a phase reference part having a predetermined period of time in total is provided at the front and/or rear of a time slot of the digital pulse signal and the phaes of the pulse sigal is changed to a predetermined value according to a predetermined monotone function in the first half of the remaining period of time of the time slot, and then changed later in the time slot in the opposite direction to a reference value, with the phase change in the first half of the remaining period time being effected in a first, e.g., increasing, direction in correspondence to a "mark" state of the signal and in a second opposite, e.g., decreasing, direction in correspondence to a "space" state, and the signal thus transmitted is received and divided into two parts and one of the two parts is compared in phase with the other after being delayed, to obtain the original digital pulse signal.
In the just-described system, the reference phase part is so determined as to occupy a half (1/2) or quarter (1/4) of one time slot of the digital pulse signal to perform the delay detection. However, in such a modem system, as the carrier frequency variation .DELTA..OMEGA. increases, the variation .DELTA..OMEGA..times.T/2 or .DELTA..OMEGA..times.T/4 of the referece operating point of the phase comparator increases so greatly that the stability of the delay detection is not considerably improved. Therefore, the carrier frequency should be so determined that the stability is not adversely affected by the variation .DELTA..OMEGA..
Also, in the demodulated signal obtained in the modem system described in Japanese Patent Application No. 165014/86, the positive phase part is shifted by 1/2 time slot from the negative phase part. Therefore, for instance, in the case where, as shown in FIG. 17, "mark" signals occur successively, they may be mistaken for a succession of "space" signals if the timing is shifted only a 1/2 time slot. Where, in contrast, "space" signals occur successively, they may be mistaken for a succession of "mark" signals.
This difficulty may be overcome by transmitting a predetermined signal train on the transmitting side, and detecting the signal train on the receiving side to determine the timing, so that the demodulated signal is accurately identified. However, the method is not suitable for high-speed communication, because it is necessary to transmit the signal required for the determination of the timing in addition to the data which are originally to be transmitted, with the result that the data transmission density is decreased. Furthermore, on the signal receiving side, it is necessary to provide a processing time for determination of the timing. Also, the same signal must be trasmitted several times as the case may be, which further increases the data transmission time.